Publications

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We present a material frame formulation analogous to the spatial frame formulation developed by Hardy, whereby expressions for continuum mechanical variables such as stress and heat flux are derived from atomic-scale quantities intrinsic to molecular simulation. This formulation is ideally suited for developing an atomistic-to-continuum correspondence for solid mechanics problems. We derive expressions for the first Piola-Kirchhoff (P-K) stress tensor and the material frame heat flux vector directly from the momentum and energy balances using localization functions in a reference configuration. The resulting P-K stress tensor, unlike the Cauchy expression, has no explicit kinetic contribution. The referential heat flux vector likewise lacks the kinetic contribution appearing in its spatial frame counterpart. Using a proof for a special case and molecular dynamics simulations, we show that our P-K stress expression nonetheless represents a full measure of stress that is consistent with both the system virial and the Cauchy stress expression developed by Hardy. We also present an expanded formulation to define continuum variables from micromorphic continuum theory, which is suitable for the analysis of materials represented by directional bonding at the atomic scale. © 2009 Elsevier Inc.

Past experimental efforts to improve CZT crystals for gamma spectrometer applications have been focused on reducing micron-scale defects such as tellurium inclusions and precipitates. While these micron-scale defects are important, experiments have shown that the micron-scale variations in transport can be caused by the formation and aggregation of atomic-scale defects such as dislocations and point defect clusters. Moreover, dislocation cells have been found to act as nucleation sites that cause the formation of large precipitates. To better solve the uniformity problem of CZT, atomic-scale defects must be understood and controlled. To this end, we have begun to develop an atomistic model that can be used to reveal the effects of small-scale defects and to guide experiments for reducing both atomic- and micron-scale (tellurium inclusions and precipitates) defects. Our model will be based upon a bond order potential (BOP) to enable large-scale molecular dynamics simulations of material structures at a high-fidelity level that was not possible with alternative methods. To establish how BOP improves over existing approaches, we report here our recent work on the assessment of two representative literature CdTe interatomic potentials that are currently widely used: the Stillinger-Weber (SW) potential and the Tersoff-Rockett (TR) potential. Careful examinations of phases, defects, and surfaces of the CdTe system were performed. We began our study by using both potentials to evaluate the lattice constants and cohesive energies of various Cd, Te, and CdTe phases including dimer, trimer, chain, square, rhomboid, tetrahedron, diamond-cubic (dc), simple-cubic (sc), body-centered-cubic (bcc), face-centered cubic (fcc), hexagonal-close-packed (hcp), graphite-sheet, A8, zinc-blende (zb), wurtzite (wz), NaCl, CsCl, etc. We then compared the results with our calculations using the density functional theory (DFT) quantum mechanical method. We also evaluated the suitability of the two potentials to predict the surface reconstructions and surface energies, various defect configurations and defect energies (interstitials and voids), elastic constants, and melting temperatures of different phases. We found that both potentials predicted incorrect energy trends as compared with those predicted by the DFT method. Most seriously, both potentials predicted incorrect lowest energy phases. These studies clearly showed that the existing potentials are not sufficient for correctly predicting the charge transport properties of CdTe demonstrating the need for a new potential. We anticipate that our BOP method will overcome this problem and will accelerate the discovery of a synthesis approach to produce improved CZT crystals.

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This research utilizes a method for calculating an atomic-scale deformation gradient within the framework of continuum mechanics using atomistic simulations to examine bicrystal grain boundaries subjected to shear loading. We calculate the deformation gradient, its rotation tensor from polar decomposition, and estimates of lattice curvature and vorticity for thin equilibrium bicrystal geometries deformed at low temperature. These simulations reveal pronounced deformation fields that exist in small regions surrounding the grain boundary, and demonstrate the influence of interfacial structure on mechanical behavior for the thin models investigated. Our results also show that more profound insight is gained concerning inelastic grain boundary phenomena by analyzing the deformed structures with regard to these continuum mechanical metrics.

Understanding charge transport processes at a molecular level using computational techniques is currently hindered by a lack of appropriate models for incorporating anistropic electric fields in molecular dynamics (MD) simulations. An important technological example is ion transport through solid-electrolyte interphase (SEI) layers that form in many common types of batteries. These layers regulate the rate at which electro-chemical reactions occur, affecting power, safety, and reliability. In this work, we develop a model for incorporating electric fields in MD using an atomistic-to-continuum framework. This framework provides the mathematical and algorithmic infrastructure to couple finite element (FE) representations of continuous data with atomic data. In this application, the electric potential is represented on a FE mesh and is calculated from a Poisson equation with source terms determined by the distribution of the atomic charges. Boundary conditions can be imposed naturally using the FE description of the potential, which then propagates to each atom through modified forces. The method is verified using simulations where analytical or theoretical solutions are known. Calculations of salt water solutions in complex domains are performed to understand how ions are attracted to charged surfaces in the presence of electric fields and interfering media.

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Non-destructive detection methods can reliably certify that gas transfer system (GTS) reservoirs do not have cracks larger than 5%-10% of the wall thickness. To determine the acceptability of a reservoir design, analysis must show that short cracks will not adversely affect the reservoir behavior. This is commonly done via calculation of the J-Integral, which represents the energetic driving force acting to propagate an existing crack in a continuous medium. J is then compared against a material's fracture toughness (J{sub c}) to determine whether crack propagation will occur. While the quantification of the J-Integral is well established for long cracks, its validity for short cracks is uncertain. This report presents the results from a Sandia National Laboratories project to evaluate a methodology for performing J-Integral evaluations in conjunction with its finite element analysis capabilities. Simulations were performed to verify the operation of a post-processing code (J3D) and to assess the accuracy of this code and our analysis tools against companion fracture experiments for 2- and 3-dimensional geometry specimens. Evaluation is done for specimens composed of 21-6-9 stainless steel, some of which were exposed to a hydrogen environment, for both long and short cracks.

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Materials with characteristic structures at nanoscale sizes exhibit significantly different mechanical responses from those predicted by conventional, macroscopic continuum theory. For example, nanocrystalline metals display an inverse Hall-Petch effect whereby the strength of the material decreases with decreasing grain size. The origin of this effect is believed to be a change in deformation mechanisms from dislocation motion across grains and pileup at grain boundaries at microscopic grain sizes to rotation of grains and deformation within grain boundary interface regions for nanostructured materials. These rotational defects are represented by the mathematical concept of disclinations. The ability to capture these effects within continuum theory, thereby connecting nanoscale materials phenomena and macroscale behavior, has eluded the research community. The goal of our project was to develop a consistent theory to model both the evolution of disclinations and their kinetics. Additionally, we sought to develop approaches to extract continuum mechanical information from nanoscale structure to verify any developed continuum theory that includes dislocation and disclination behavior. These approaches yield engineering-scale ex-pressions to quantify elastic and inelastic deformation in all varieties of materials, even those that possess highly directional bonding within their molecular structures such as liquid crystals, covalent ceramics, polymers and biological materials. This level of accuracy is critical for engineering design and thermo-mechanical analysis is performed in micro- and nano systems. The research proposed here innovates on how these nanoscale deformation mechanisms should be incorporated into a continuum mechanical formulation, and provides the foundation upon which to develop a means for predicting the performance of advanced engineering materials.

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To effectively integrate nanotechnology into functional devices, fundamental aspects of material behavior at the nanometer scale must be understood. Stresses generated during thin film growth strongly influence component lifetime and performance; stress has also been proposed as a mechanism for stabilizing supported nanoscale structures. Yet the intrinsic connections between the evolving morphology of supported nanostructures and stress generation are still a matter of debate. This report presents results from a combined experiment and modeling approach to study stress evolution during thin film growth. Fully atomistic simulations are presented predicting stress generation mechanisms and magnitudes during all growth stages, from island nucleation to coalescence and film thickening. Simulations are validated by electrodeposition growth experiments, which establish the dependence of microstructure and growth stresses on process conditions and deposition geometry. Sandia is one of the few facilities with the resources to combine experiments and modeling/theory in this close a fashion. Experiments predicted an ongoing coalescence process that generates signficant tensile stress. Data from deposition experiments also support the existence of a kinetically limited compressive stress generation mechanism. Atomistic simulations explored island coalescence and deposition onto surfaces intersected by grain boundary structures to permit investigation of stress evolution during later growth stages, e.g., continual island coalescence and adatom incorporation into grain boundaries. The predictive capabilities of simulation permit direct determination of fundamental processes active in stress generation at the nanometer scale while connecting those processes, via new theory, to continuum models for much larger island and film structures. Our combined experiment and simulation results reveal the necessary materials science to tailor stress, and therefore performance, in nanostructures and, eventually, integrated nanocomponents.